In August 1914 he went to France to help with the work of the American Field Service - which provide ambulances and drivers to support French and British forces. He was also attached to the British Army Medical Corps and assisted with the establishment of a typhoid hospital near Ypres. His book For Better Relations with Our Latin American Neighbors was published in 1915. He was then commissioned a major in the U.S. Army in May 1917 before sailing to France as a member of General Pershing's staff. He was promoted to lieutenant colonel in 1918 and served as Chief of the American Military Mission at British General Headquarters. He returned to the United States in April 1919.

Bacon died on May 29, 1919 from blood poisoning after undergoing surgery on his mastoiditis.[1]